Ice melt is continuing around the world as temperatures warm and as glaciers, and ice sheets slowly disappear. Now, researchers have underscored the need for continuous satellite monitoring of ice sheet melt in order to better identify and predict melting and corresponding sea level rise.

Ice sheets cover vast swathes of land and water in Antarctica and Greenland, containing about 99.5 percent of the Earth's glacier ice. If they melted completely, global sea levels would rise about 200 feet--something that would have disastrous consequences for coastal cities and countries. Because of this, these ice sheets are the largest potential source for sea level rise. At the same time, though, their future behavior is largely uncertain; it's difficult to predict their responses using numerical monitoring, which is why researchers have turned to extrapolating observed changes to estimate the contribution to sea level rise in the future. The only problem with this method, though, is the fact that scientists need to continuously monitor the ice sheets.

Since 2002, the satellites of the Gravity Recover and Climate Experiment (GRACE) have detected tiny variations in Earth's gravity field resulting from changes in mass distribution, including movement of ice into the oceans. Using these changes in gravity, researchers can monitor the state of the ice sheets at monthly intervals.

In order to examine exactly how fast the ice sheets were melting, the researchers compared nine years of satellite data from the GRACE mission with reconstructions of about 50 years of mass changes to the ice sheet. In the end, they found that accurately detecting an accelerating trend in melting depended on the length of the record.

"In the course of the mission, it has become apparent that ice sheets are losing substantial amounts of ice--about 300 billion tons each year--and that the rate at which these losses occurs is increasing," said Bert Wouters, a visiting researcher at the University of Colorado, in a news release. "Compared to the first few years of the GRACE mission, the ice sheets' contribution to sea level rise has almost doubled in recent years."

So how fast are we losing ice and gaining water? It turns out that the ice loss is larger than expected to see from natural fluctuations, but the speed-up of ice loss over the last years is not.

So what does this mean exactly? It shows the importance of continuous satellite monitoring. There may be enough satellite data to detect a speed-up in mass loss in the Antarctic ice sheet, but another ten years of data is needed to do the same for Greenland.